This invention relates generally to electrostatography, and more particularly it relates to improvements in developer liquid supplying devices in electrostatographic copying systems employing liquid development techniques.
In a known electrostatographic copying system a charge pattern is established on an imaging surface and is developed by a liquid development process wherein the liquid developer is presented to the charge pattern by an applicator or liquid supplying device which has a surface comprising raised areas or portions ("lands") and recessed portions or depressions ("valleys") adapted to contact liquid developer between or among the raised portions. The liquid developer is drawn to the imaging surface in image configuration by the electrostatic forces of the charge pattern.
Preferred methods and devices for the liquid development of electrostatic charge patterns are shown in U.S. Pat. No. 3,084,043 and U.S. Pat. No. 3,806,354. In these systems a developer liquid supplying member, generally an applicator roll, is utilized to present liquid developer to the surface of the member carrying the charge pattern. The applicator roll is normally prepared for coming into developing contact with the imaging surface by first contacting it with a doctoring or metering blade or other resilient or absorbent metering device or combinations thereof, in order to adjust the liquid in the valleys to a predetermined level and to clean the lands so that they are substantially free from liquid developer.
In the foregoing system a liquid developer supply member having on the surface thereof finely divided raised portions and depressions in a regular geometric pattern, is used to develop a latent image by the steps of supplying a liquid developer to the depressions of the developer supply member and bringing the raised portions of the developer supply member into contact with a latent image supporting member bearing an electrostatic latent image so as to cause the electrically conductive developer liquid retained in the depressions to creep up along the side of the raised portions by the attraction of the charges of the latent image into contact with the latent image. Undesirable amounts of liquid developer in the depressions results in the deposit of liquid developer in the background, and this is caused by the inability of the prior art metering devices both to meter the depth of the liquid developer at proper levels in the applicator valleys and to render the lands substantially clean. The volume of liquid developer which must be pushed aside by or absorbed by the prior art metering devices is frequently more than the device can handle and still clean the lands, or the absorbent type of metering device can only meter to the extent of its absorbing capacity without the use of additional means to clean liquid developer from the absorbing device. Furthermore, the absorbing type doctoring devices, even when used in conjunction with a resilient or rigid land cleaning member, tend to remove too much liquid developer from the depressions or too little liquid developer from the depressions or both, depending upon the absorbency of the absorbent material. Representative of these prior art devices are the disclosures in U.S. Pat. No. 3,830,199 and U.S. Pat. No. 3,667,428.
In the foregoing liquid development methods and devices there is also a risk of the surface of the latent image supporting member being damaged due to contact with the developer supplying member. In order to avoid this damage, liquid developer supplying members formed from a resilient, compressible electrically conductive material are used, and excess developer liquid supplied in the depressions is wiped or scraped therefrom by a doctor blade or similar member to leave a desired amount of developer in the depressions.
In these devices and methods, the liquid developer c remains within the depressions b of the liquid developer supplying member a after doctoring, and liquid developer does not adhere to the rigid raised portions d, that is, the position in which the level of the developer is maintained below peaks of the raised portions by a distance l as illustrated in FIG. 1. The value of the distance l, together with the electrostatic attractive force of the latent image supporting member, may affect development greatly. In particular, a residual developer liquid remaining at the raised portions d transfers to the imaging member and results in dirtying the background areas of the image. For this reason, in the aforementioned method in which the excess developer is removed or wiped off with a doctor blade or similar member, the wiper or scraper member e is made of a resilient material and in such a configuration that during wiping or scraping part of the wiper e is deformed to project toward the bottom of the depressions b of the developer liquid supply member a as shown in FIG. 2. To this end, the depressions b of the supply member a are formed in the form of a spiral having rigid, non-compressible raised areas or lands as illustrated in FIG. 3.
However, the use of a liquid developer supply member a having spiral depressions b for liquid developing a latent image has inherently presented a disadvantage in that, where an electrostatic latent image on the latent image supporting member f lies across the depression b as shown in FIG. 4, the developer liquid is caused to adhere merely to the portions of the latent image supporting member f adjacent the peaks of raised portions d as illustrated in FIG. 5 so that areas of the latent image placed between the raised portions d and above the depression b remain undeveloped, thereby resulting in image discontinuity as shown in FIG. 6. Furthermore, developer supply members having spiral depressions as in FIG. 3 are disadvantageous in that the flow of the developer liquid is controllable only in the direction perpendicular to that of the spiral depressions while the flow control in other directions is impossible. Therefore, the sharpness of developed image in the direction perpendicular to that of the depressions is of low quality.
Although satisfactory developed images can be obtained using the metering blades, rollers and other devices of the prior art, an alternative and improved means for preparing a liquid developer supplying device or applicator surface for coming into developing contact with an imaging surface is desirable, especially in those systems where the liquid supplying member is one having at least compressible, resilient raised areas or lands.